Micromechanical devices with oscillatably suspended oscillation elements may be employed both as micromechanical sensors and as micromechanical actuators. The micromechanical device consisting of the spring portion and the oscillatably suspended oscillation element has an eigenfrequency or resonance frequency. In many applications, the resonance frequency of the micromechanical device is to correspond to a fixedly default frequency, in order to achieve, using the resonance increase, for example sufficient sensitivity in the case of a sensor and for example sufficient oscillation amplitude in the case of an actuator. Examples for micromechanical structures e.g. are those serving as clock transmitters in e.g. clocks, or deflecting elements such as scanner mirrors, which are used for data projection, wherein the data frequency and the oscillation frequency is to be in an fixedly default ratio. So as to keep the power needed for the oscillation generation low, such devices generally possess relatively high quality, with the result that the resonance curve is narrow and there is only very little margin in the excitation frequency when maintaining the desired oscillation amplitude.
In MEMS (micro electro mechanical systems) scanner mirrors, even slight variations of the width of the torsion springs significantly change the resonance frequencies. Fabrication variations of the spring width may be due to the etching process or the photolithographic resist mask. Depending on the spring geometry to be fabricated, these variations have some influence on the spring hardness, and hence on the oscillation frequency of the device. For example, this is especially critical in devices for two-dimensional deflection of light, such as the MEMS scanner mirrors, since here often a certain fixed ratio of the oscillation frequencies is needed. An MEMS scanner, for example, is described in the doctoral thesis “Ein neuartiger Mikroaktor zur ein-und zweidimensionalen Ablenkung von Licht” by Schenk, Gerhard-Mercator-Gesamthochschule Duisburg, 2000.
Above all, the systematic deviations of the spring dimensions have great influence on the frequency ratio of such a micromechanical oscillator. It is strongly influenced by variations of the fabrication process. Usually, movable parts are defined with the aid of etching processes in the Microsystems technology. Here, the properties of the masks and the etching processes employed have an influence on the types and order of magnitude of the variations. It is distinguished between global, local and direction-dependent variations.
Global variations influence the geometries of all devices fabricated in a step. One example for a global variation is the time-dependent variation of the pressure in the etching gas.
Local variations influence the dimensions of the fabricated geometries in location-dependent manner. The location-dependent variation of the concentration of the etching gas in the process chamber is one example for a local variation.
Direction-dependent variations influence the dimension of the fabricated geometry depending on its orientation in the process chamber or with respect to the chamber center.
On the left-hand side, FIG. 4 shows a greatly simplified illustration of a micromechanical oscillator 400 with an oscillating body 410 and the springs 420 and 422. It can be seen that both the geometry of the springs 420, 422 and the geometry of the oscillating body 410 are defined by so-called open trenches 440. On the right-hand side, FIG. 4 shows the detail of the spring 420 with an etched trench 440 on both sides of the spring 420. These open trenches are produced by a dry etching process, for example. Such a process is characterized in that fabrication variations mainly occur in the trench width and in the vertical profile of the trench. Shifts of the trenches with respect to each other are negligible. Both deviations of the spring geometry and deviations of the geometry of the oscillating body result from the variations of the trench geometry.
For the correction of the resonance frequency, and hence the frequency ratio, there are various approaches. In one implementation, the ambient pressure, and hence the effective mass, of the moved element is altered by application of gas (U.S. Pat. Nos. 6,331,909, 6,285,489). The apparatus needed for this and the regulating circuit, however, are relatively intensive. In the same patent specification, there is presented a second method, in which the spring is covered with a gas-absorbing material. Upon absorption, the material properties of the spring change, and hence the frequency. The outlay for this method also seems relatively high. Moreover, it is to be assumed that the quality of the system is diminished by the use of an absorbing material for the springs.
In another implementation (U.S. Pat. Nos. 6,256,131, 6,285,489), in torsion oscillators, part of the rotating mass may be shifted toward or away from the torsion axis by means of electrostatic forces. Thereby, the moment of inertia, and hence again the resonance frequency, changes. Regulation of the resonance frequency may indeed be done thereby, but greater variations cannot be corrected due to the generally small translation paths of the moveable mass.
Since the regulation range of the resonance frequency in micromechanical devices generally is small as compared with fabrication variations, sorting substantially takes place such that devices with too large a deviation cannot be used. Thus, the yield is reduced significantly.
With the aid of geometrical structures, which are broken by external influence in targeted manner, the effective length, and hence the stiffness, of micromechanical spring elements can be influenced.
This solution was already filed by the Fraunhofer Institute for Photonic Microsystems in the patent application with the international publication number WO 2004/092745 A1.
Through the equipment of the oscillating body with additional structures, so-called compensation trenches, the mass or mass moment of inertia of the oscillator may be influenced, so that fabrication-induced deviation of the spring geometry can be compensated for at least partially. This solution was filed with the German Patent and Trademark Office by the Fraunhofer Institute for Photonic Microsystems under the application number 102007001516.1-54.
With the aid of a spring elements that can be influenced in their geometry, the spring hardness, and hence the resonance frequency, can be adjusted within certain boundaries. This solution was filed with the German Patent and Trademark Office by the Fraunhofer Institute for Photonic Microsystems under the application number 102007001516.1.
An efficient way for targeted adjustment or tuning of the sensitivity of the spring hardness to fabrication variations in such micromechanical devices therefore is desirable.